Cooperative activation of IP3 receptors by sequential binding of IP3 and Ca2+ safeguards against spontaneous activity

BACKGROUND Ca2+ waves allow effective delivery of intracellular Ca2+ signals to cytosolic targets. Propagation of these regenerative Ca2+ signals probably results from the activation of intracellular Ca2+ channels by the increase in cytosolic [Ca2+] that follows the opening of these channels. Such positive feedback is potentially explosive. Mechanisms that limit the spontaneous opening of intracellular Ca2+ channels are therefore likely to have evolved in parallel with the mechanism of Ca2+-induced Ca2+ release. RESULTS Maximal rates of 45Ca2+ efflux from permeabilised hepatocytes superfused with medium in which the [Ca2+] was clamped were cooperatively stimulated by inositol 1,4,5-trisphosphate (IP3). A minimal interval of approximately 400 msec between IP3 addition and the peak rate of Ca2+ mobilisation indicate that channel opening does not immediately follow binding of IP3. Although the absolute latency of Ca2+ release was unaffected by further increasing the IP3 concentration, it was reduced by increased [Ca2+]. CONCLUSIONS We propose that the closed conformation of the IP3 receptor is very stable and therefore minimally susceptible to spontaneous activation; at least three (probably four) IP3 molecules may be required to provide enough binding energy to drive the receptor into a stable open conformation. We suggest that a further defence from noise is provided by an extreme form of coincidence detection. Binding of IP3 to each of its four receptor subunits unmasks a site to which Ca2+ must bind before the channel can open. As IP3 binding may also initiate receptor inactivation, there may be only a narrow temporal window during which each receptor subunit must bind both of its agonists if the channel is to open rather than inactivate.